From b0c6feb73ff2b97960f537396362811ce4ec9c00 Mon Sep 17 00:00:00 2001
From: Nathaniel Walizer <robots.are.love@gmail.com>
Date: Tue, 18 Mar 2025 18:45:55 -0700
Subject: [PATCH] Fix sprite priority

---
 src/ppu.c | 94 +++++++++++++++++++++----------------------------------
 1 file changed, 36 insertions(+), 58 deletions(-)

diff --git a/src/ppu.c b/src/ppu.c
index ec5c697..6079195 100644
--- a/src/ppu.c
+++ b/src/ppu.c
@@ -232,6 +232,9 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
 
         int sprite_bank = !!(mem->ctrl & ppu_Control_Sprite_Bank) << 2;
 
+        uint8_t foreground[nes_ppu_render_w];
+        memset(foreground, 0xFFU, nes_ppu_render_w);
+
         int n_sprites = 0;
         const oam_sprite* sprites = (oam_sprite*)mem->oam;
         const oam_sprite* sprite = sprites +
@@ -265,7 +268,10 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
             int pat1 = ((pl0 >> 1) & 0x55) | (pl1 & 0xAA);
 
             const uint8_t* pal = &mem->palette[0x10 + ((sprite->attr & oam_Attr_Pal_Mask) << 2)];
-            uint8_t* dst = scanline_ptr + sprite->x;
+            const uint8_t* back =
+                    (sprite->attr & oam_Attr_Background) ?
+                    (scanline_ptr + sprite->x) : NULL;
+            uint8_t* dst = foreground + sprite->x;
 
             const int over_x = ((int)sprite->x + 8) - nes_ppu_render_w;
 
@@ -275,73 +281,31 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
                 pat1 = sprite_rev[tmp];
             }
 
-            // TODO: Handle column 0 sprite mask
+#define do_sprite(idx, pi, shift)                           \
+                if (back && back[idx] != 0xFFU) {           \
+                    dst[idx] = back[idx];                   \
+                } else {                                    \
+                    int pal_idx = (pat##pi >> shift) & 3;   \
+                    if (pal_idx) dst[idx] = pal[pal_idx];   \
+                }
 
             switch (over_x) {
             default:
-            {   int pal_idx = (pat0 >> 0) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[7] == 0xFFU)
-                        && pal_idx) {
-                    dst[7] = pal[pal_idx];
-                }
-            }
+                do_sprite(7, 0, 0);
             case 1:
-            {   int pal_idx = (pat1 >> 0) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[6] == 0xFFU)
-                        && pal_idx) {
-                    dst[6] = pal[pal_idx];
-                }
-            }
+                do_sprite(6, 1, 0);
             case 2:
-            {   int pal_idx = (pat0 >> 2) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[5] == 0xFFU)
-                        && pal_idx) {
-                    dst[5] = pal[pal_idx];
-                }
-            }
+                do_sprite(5, 0, 2);
             case 3:
-            {   int pal_idx = (pat1 >> 2) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[4] == 0xFFU)
-                        && pal_idx) {
-                    dst[4] = pal[pal_idx];
-                }
-            }
+                do_sprite(4, 1, 2);
             case 4:
-            {   int pal_idx = (pat0 >> 4) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[3] == 0xFFU)
-                        && pal_idx) {
-                    dst[3] = pal[pal_idx];
-                }
-            }
+                do_sprite(3, 0, 4);
             case 5:
-            {   int pal_idx = (pat1 >> 4) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[2] == 0xFFU)
-                        && pal_idx) {
-                    dst[2] = pal[pal_idx];
-                }
-            }
+                do_sprite(2, 1, 4);
             case 6:
-            {   int pal_idx = (pat0 >> 6) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[1] == 0xFFU)
-                        && pal_idx) {
-                    dst[1] = pal[pal_idx];
-                }
-            }
+                do_sprite(1, 0, 6);
             case 7:
-            {   int pal_idx = (pat1 >> 6) & 3;
-                if (    (   !(sprite->attr & oam_Attr_Background) ||
-                            dst[0] == 0xFFU)
-                        && pal_idx) {
-                    dst[0] = pal[pal_idx];
-                }
-            }
+                do_sprite(0, 1, 6);
             }
 
             ++n_sprites;
@@ -352,6 +316,20 @@ void nes_ppu_render_line(nes_PPU* ppu, nes_PPU_Memory* mem) {
         } else {
             mem->status &= ~ppu_Status_Overflow;
         }
+
+        // Mask column 0?
+        if (!(mem->mask & ppu_Mask_Left_Sprite)) {
+            memset(foreground, 0xFFU, 8);
+        }
+
+        // Overlay the sprites
+        const uint8_t* fg = foreground;
+        uint8_t* dst = scanline_ptr;
+        for (int i = 0; i < nes_ppu_render_w; ++i) {
+            if (*fg != 0xFFU) *dst = *fg;
+            ++fg;
+            ++dst;
+        }
     }
 
     // Increment internal registers